Muscle Tissue Types

Muscle Tissue Muscle Tissue Types

Muscle tissue is characterized by properties that allow movement. Muscle cells are excitable meaning that they can respond to a stimulus. They are also contractile, meaning they can shorten and generate a force. When attached between two movable objects (bones) muscle contractions cause the bones to move. Some muscle movement is voluntary, which means it is under conscious control. For example, when a person uses their arm to open a book and read a chapter on anatomy. Other movements are involuntary, meaning they are not under conscious control, such as the change in the diameter of your pupil in response to bright light. Muscle tissue is classified into three types according to structure and function: skeletal, cardiac, and smooth.

Skeletal muscle is attached to bones and its contraction makes possible locomotion, facial expressions,

posture, and other voluntary movements of the body. Forty percent of your body mass is made up of skeletal muscle. Another function of skeletal muscle is to generate heat as a byproduct of their contraction and thus participate in thermal homeostasis. Shivering is an involuntary contraction of skeletal muscles in response to perceived lower than normal body temperature. Under a light microscope, muscle cells appear striated with many nuclei squeezed along the membranes. The striation is due to the regular alternation of the contractile proteins actin and myosin, along with the structural proteins that couple the contractile proteins to connective tissues. The cells are multinucleated as a result of the fusion of the many myoblasts that fuse to form each long muscle fiber. The gross anatomy of skeletal muscle is considered further below.

Cardiac muscle forms the contractile walls of the heart. The cells of cardiac muscle, known as cardiomyocytes, also appear striated under the microscope. Unlike skeletal muscle fibers, cardiomyocytes are single, branched cells typically with a single centrally located nucleus. Cardiomyocytes attach to one another with specialized cell junctions called intercalated discs which have both anchoring junctions and gap junctions. Attached cells form long, branching cardiac muscle fibers that are, essentially, a mechanical and electrochemical syncytium allowing the cells to synchronize their actions.

Types

Smooth muscle tissue contraction is responsible for involuntary movements in the internal organs. It forms the contractile component of the digestive, urinary, and reproductive systems as well as the airways and arteries. Each cell is small, spindle-shaped, has a single nucleus, and no visible striations.